An effective low-bit-rate speech coder should have the characteristics of high speech intelligibility, speaker independence, ease of real time implementation and short throughput delay. To maintain low-bit-rate transmission and simultaneously achieve these goals is conventionally considered contradictory.
Various speech encoding algorithms and techniques have been proposed for encoding and decoding low-data-rate speech parameters from and to speech signals. Techniques for vector quantization of line spectrum pairs (LSP) data converted from standard linear predictive coding (LPC) parameters derived from input speech signals has been suggested, for example, in "Application of Line-Spectrum Pairs to Low-Bit-Rate Speech Encoders", by G. S. Kang and L. J. Fransen, Naval Research Laboratory, at Proceedings ICASSP, 1985, pp. 244-247. A tree-encoding technique using adaptive or time-varying quantization was disclosed by N. S. Jayant and S. A. Christensen, Bell Laboratories, at IEEE Transactions on Communications, COM-26, September 1978, pp. 1376-1379. For transmitted speech signals encoded by vector quantization, an improvement in decoding performance at the receiver end by optimization of the codebook for decoding words from the incoming signals was disclosed in "Improving The Codebook Design For Vector Quantization", by Y. J. Liu, ITT Defense Communication Division. at Proceedings IEEE Military Communications, 1987, pp. 556-559.
However, the prior art proposals leave much to be desired in terms of achieving high intelligibility and reliability at low data rates, e.g. 400 bits-per-second (bps), and particularly for speaker independent speech coding in communications over a high frequency (HF) channel in a hostile environment. In the prior systems, LSP coefficients are computed by fast Fourier transform (FFT). Also, the group delay function is utilized as weighting coefficients to compensate for distortion in the vector quantization. The latter is a time consuming process which may require even more time than that required to compute the LSP coefficients.